Python One Line While Loop [A Simple Tutorial]

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Python is powerful — you can condense many algorithms into a single line of Python code. So the natural question arises: can you write a while loop in a single line of code? This article explores this mission-critical question in all detail.

How to Write a While Loop in a Single Line of Python Code?

There are three ways of writing a one-liner while loop:

  • Method 1: If the loop body consists of one statement, write this statement into the same line: while True: print('hi'). This prints the string 'hi' to the shell for as long as you don’t interfere or your operating system forcefully terminates the execution.
  • Method 2: If the loop body consists of multiple statements, use the semicolon to separate them: while True: print('hi'), print('bye'). This runs the statements one after the other within the while loop.
  • Method 3: If the loop body consists nested compound statements, replace the inner compound structures with the ternary operator: while True: print('hi') if condition else print('bye').

Exercise: Run the code. What do you observe? Try to fix the infinite loop!

Next, you’ll dive deep into each of these methods and become a better coder in the process.

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But enough promo, let’s dive into the first method—the profane…

Method 1: Single-Statement While Loop One-Liner

Just writing the while loop into a single line of code is the most direct way of accomplishing the task. Say, you want to write the following infinite while loop in a single line of code:

while True:

You can easily get this done by writing the command in a single line of code:

# Method 1: Single-Line While Loop
while True: print('hi')

While this answer seems straightforward, the interesting question is: can we write a more complex while loop that has a longer loop body in a single line?

Related Article: If you’re interested in compressing whole algorithms into a single line of code, check out this article with 10 Python one-liners that fit into a single tweet.

Let’s explore an alternative Python trick that’s very popular among Python masters:

Method 2: Multi-Statement While Loop One-Liner

As it turns out, you can also use the semicolon to separate multiple independent statements and express them in a single line. The statement expression1; expression2 reads “first execute expression1, then execute expression2.

Here’s an example how you can run a while loop until a counter variable c reaches the threshold c == 10:

c = 0
while c < 10: print(c); c = c + 1

This way, you can easily compress “flat” loop bodies in a single line of Python code.

But what if the loop body is not flat but nested in a hierarchical manner—how to express those nested while loops in a single line?

Method 3: Nested Compound Statements While Loop One-Liner

You often want to use compound statements in Python that are statements that require an indented block such as if statements or while loops.

In the previous methods, you’ve seen simple while loop one-liners with one loop body statement, as well as multiple semicolon-separated loop body statements.

Problem: But what if you want to use a compound statement within a simple while loop—in a single line of code?

Example: The following statement works just fine:

# YES:
if expression: print('hi')

You can also add multiple statements like this:

# YES:
if expression: print('hi'); print('ho')

But you cannot use nested compound statements in a while loop one-liner:

# NO:
while expression1: if expression2: print('hi')

Python throws an error does not work because both the while and if statements are compound.

Nested Compound Statements Error

However, there’s an easy fix to make this work. You can replace the if expression2: print('hi') part with a ternary operator and use an expression rather than a compound statement:

# Method 3: One-Line While Loop + Ternary Operator
while True: print('yes') if True else print('no')

You can also use nested ternary operators to account for possibly nested if blocks:

Python Ternary Elif

Related Video: One-Line For Loop

Python One Line For Loop [A Simple Tutorial]

You can find out more about the single-line for loop in my detailed article here.

Where to Go From Here

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